This application claims the priority of Korean Patent Application No. 2002-67964, filed on Nov. 4, 2002, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a method of controlling upstream data transmission in an Ethernet passive optical network (hereinafter, referred to as an “EPON”) and an apparatus thereof.
2. Description of the Related Art
Recently, a passive optical network (PON) based on low-cost and high-efficiency Ethernet has come to forefront as the network configuration technology for high-speed optical subscriber networks. An EPON serves as the transmission network between a subscriber and a backbone network. An EPON has the structure of an optical subscriber network, in which several optical network units (ONUs) are connected to an optical line termination (OLT) in a tree-topological branch structure using a passive optical splitter. In an EPON, due to physical connection characteristic of the tree-topology, downstream data transmission and upstream data transmission are performed in different ways. Downstream data from an external network to the subscriber is broadcasted from an OLT to all ONUs. On the other hand, upstream data transmission from the subscriber to the external network requires an efficient transmission control (medium access control (MAC)) function, so that data from each ONU does not collide after being sent to the OLT.
An OLT and an ONU in EPON system communicate with an Ethernet protocol data unit of IEEE 802.3 MAC frame. The IEEE 802.3ah, study groups of standardization for Ethernet in the First Mile (EFM), recommend multi-point control protocol (MPCP) as a method for controlling PON transmission while preserving the Ethernet MAC protocol. MPCP is a transmission control protocol which controls transmission between ONUs and an OLT via a MAC control frame, in a higher layer than the 802.3 MAC control layer. Basic functions of MPCP include ONU registration and ranging functions, as well as a function for allocating upstream bandwidth. For this purpose, additional control frames such as GATE, REPORT, REGISTER, REGISTER_REQUEST and REGISTER_ACK are defined. The current MPCP has been widely recognized as a protocol for controlling upstream data transmission in an EPON. Standardization regarding an MPCP message format for efficient upstream bandwidth allocation has been discussed with enthusiasm.
For bandwidth allocation between ONUs and an OLT, MPCP implementations support a static bandwidth allocation mode (which is a simple time-sharing without request) or a dynamic bandwidth allocation mode in which an OLT grants bandwidth to each ONU through request messages. The static bandwidth allocation mode is simple to implement and guarantees constant transmission delay. However, when a system load is low, bandwidth utilization is reduced, and the mean transmission delay is increased. In the dynamic bandwidth allocation mode, upstream bandwidth can be shared efficiently depending on the network offered load, however, the transmission delay for each frame is not constant. Thus, in the case of delay sensitive real-time services, service quality may not be guaranteed. In the prior art, the bandwidth allocation process determines the transmission duration and the transmission start time simultaneously for the permitted ONUs. However, this method can not support both static bandwidth allocation function and dynamic bandwidth allocation function. In order to apply multiple bandwidth allocation function regarding on the characteristics of services and demands, each module have to check the timing information of another bandwidth allocation module. Thus, when a conventional bandwidth allocation technique is applied, implementation of each bandwidth allocation module becomes very complicated, and the probability of unexpected fragmentation during a bandwidth allocation operation, increases.